Alginates are isolated from marine brown algae. Alginate is also produced in soil bacteria such as Azotobacter vinelandii and Azotobacter crococcum and several different Pseudomonas bacteria. However, commercially available alginate is generally derived from brown algae.
Alginates are used in foodstuffs and in pharmaceutical, dental, cosmetic and other industrial products. The most common industrial applications are based on their hydrocolloidal and polyelectrolytic nature, which forms the basis for the gel-forming, thickening, stabilising, swelling and viscosity-providing properties.
Alginates are salts of alginic acid, a linear heteropolysaccharide consisting of (1.fwdarw.4) linked .beta.-D-mannuronic acid, designated as M, and .alpha.-L-guluronic acid, designated as G.
These two uronic acids have the following formulae: ##STR1##
The polymers exist as homopolymer sequences of mannuronic acid, called M blocks, homopolymer sequences of guluronic acid, called G blocks, and mixed sequences of mannuronic and guluronic acid units, so-called MG blocks or alternating blocks.
To illustrate the structure of the alginates, the following is a schematic representation of a conceivable block structure: ##STR2##
Usually, alginate contains all three types of blocks and a block mostly consists of three to thirty monomer units. The distribution of the blocks depends on the type of algae from which the alginate is isolated as well as on the age and part of the plant, for example alginate from the stalk may have a different sequence and block composition to alginate isolated from the leaves. The time of year at which the algae are harvested also affects the block composition and sequence. According to the knowledge we have today, the highest G content can be found in the stalks of old L. hyperborea. The leaf of the same species has a somewhat lower G content and shorter G blocks, but the content is still higher than most other species. Commercially available alginates usually have a G content of 25% -70%.
Sources which have a high content of M blocks are, for example, species of the brown algae genera Durvillea, Lessonia and Ascophyllum.